WHAT HAPPENS WHEN A NEEDLE IS MAGNETIZED. The reason that a needle becomes magnetic if it is rubbed over a magnet is probably this: Every molecule of iron may be an extremely tiny magnet; if it is, each molecule has a north and south pole like the needle of a compass. In an ordinary needle (or in any unmagnetized piece of iron or steel) these molecules would be facing every way, as shown in Figure 107. [Illustration: FIG. 107. Diagram of molecules in unmagnetized iron. The north and south poles of the molecules are supposed to be pointing in all directions.] [Illustration: FIG. 108. Diagram of magnetized iron. The north and south poles of the molecules are all supposed to point in the same direction.] But when a piece of steel or iron that is already magnetized is brought near the unmagnetized needle, all the north poles of the molecules of the needle are pulled in the same direction--it is almost like combing tangled hair to stroke a needle over a magnet. Then the molecules are arranged more as shown in Figure 108. When all the molecules, each of which is a tiny magnet, pull in the same direction, they make a strong magnet, and they magnetize any iron that comes near them just as they were magnetized. Steel will stay magnetized a long time; but ordinary soft iron loses magnetism almost as soon as another magnet is taken away from it,--the molecules become all disarranged again. In a later section you will find that whenever electricity flows through a wire that is coiled around a piece of iron, the iron becomes magnetized just as when it is rubbed with a magnet. _APPLICATION 47._ An explorer lost his compass. In clear weather he could tell the directions by the sun and stars, but in cloudy weather he was badly handicapped. He had with him a gun, plenty of ammunition, a sewing kit, a hunting knife, and some provisions. How could he have made a compass? INFERENCE EXERCISE Explain the following: 281. Snow turns to water in the first warm weather. 282. A person's face looks ghastly by the greenish light of a mercury-vapor lamp. 283. If a red-hot coal is touched with a cold poker, the coal turns black at the place touched. 284. Stereopticon slides are put in upside down, yet the picture on the screen is right side up. 285. If the vocal cords of your throat did not vibrate, you could not talk out loud. 286. A watch is sometimes put out of order if it is held near a magnet. 287. The water will be no higher on the inside of a leaky boat than it is on the outside. 288. A bass viol is considerably larger than a violin. 289. Ships that are used by men testing the earth's magnetism carry very sensitive compasses. Explain why such ships are made entirely of wood and brass. 290. Thunder rolls; that is, after the first peal there is a reverberating sound that becomes less and less distinct. SECTION 32. _Static electricity._